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Copy pathceval_gil.h
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270 lines (230 loc) · 8.93 KB
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/*
* Implementation of the Global Interpreter Lock (GIL).
*/
#include <stdlib.h>
#include <errno.h>
/* First some general settings */
/* microseconds (the Python API uses seconds, though) */
#define DEFAULT_INTERVAL 5000
static unsigned long gil_interval = DEFAULT_INTERVAL;
#define INTERVAL (gil_interval >= 1 ? gil_interval : 1)
/* Enable if you want to force the switching of threads at least every `gil_interval` */
#undef FORCE_SWITCHING
#define FORCE_SWITCHING
/*
Notes about the implementation:
- The GIL is just a boolean variable (gil_locked) whose access is protected
by a mutex (gil_mutex), and whose changes are signalled by a condition
variable (gil_cond). gil_mutex is taken for short periods of time,
and therefore mostly uncontended.
- In the GIL-holding thread, the main loop (PyEval_EvalFrameEx) must be
able to release the GIL on demand by another thread. A volatile boolean
variable (gil_drop_request) is used for that purpose, which is checked
at every turn of the eval loop. That variable is set after a wait of
`interval` microseconds on `gil_cond` has timed out.
[Actually, another volatile boolean variable (eval_breaker) is used
which ORs several conditions into one. Volatile booleans are
sufficient as inter-thread signalling means since Python is run
on cache-coherent architectures only.]
- A thread wanting to take the GIL will first let pass a given amount of
time (`interval` microseconds) before setting gil_drop_request. This
encourages a defined switching period, but doesn't enforce it since
opcodes can take an arbitrary time to execute.
The `interval` value is available for the user to read and modify
using the Python API `sys.{get,set}switchinterval()`.
- When a thread releases the GIL and gil_drop_request is set, that thread
ensures that another GIL-awaiting thread gets scheduled.
It does so by waiting on a condition variable (switch_cond) until
the value of gil_last_holder is changed to something else than its
own thread state pointer, indicating that another thread was able to
take the GIL.
This is meant to prohibit the latency-adverse behaviour on multi-core
machines where one thread would speculatively release the GIL, but still
run and end up being the first to re-acquire it, making the "timeslices"
much longer than expected.
(Note: this mechanism is enabled with FORCE_SWITCHING above)
*/
#include "condvar.h"
#ifndef Py_HAVE_CONDVAR
#error You need either a POSIX-compatible or a Windows system!
#endif
#define MUTEX_T PyMUTEX_T
#define MUTEX_INIT(mut) \
if (PyMUTEX_INIT(&(mut))) { \
Py_FatalError("PyMUTEX_INIT(" #mut ") failed"); };
#define MUTEX_FINI(mut) \
if (PyMUTEX_FINI(&(mut))) { \
Py_FatalError("PyMUTEX_FINI(" #mut ") failed"); };
#define MUTEX_LOCK(mut) \
if (PyMUTEX_LOCK(&(mut))) { \
Py_FatalError("PyMUTEX_LOCK(" #mut ") failed"); };
#define MUTEX_UNLOCK(mut) \
if (PyMUTEX_UNLOCK(&(mut))) { \
Py_FatalError("PyMUTEX_UNLOCK(" #mut ") failed"); };
#define COND_T PyCOND_T
#define COND_INIT(cond) \
if (PyCOND_INIT(&(cond))) { \
Py_FatalError("PyCOND_INIT(" #cond ") failed"); };
#define COND_FINI(cond) \
if (PyCOND_FINI(&(cond))) { \
Py_FatalError("PyCOND_FINI(" #cond ") failed"); };
#define COND_SIGNAL(cond) \
if (PyCOND_SIGNAL(&(cond))) { \
Py_FatalError("PyCOND_SIGNAL(" #cond ") failed"); };
#define COND_WAIT(cond, mut) \
if (PyCOND_WAIT(&(cond), &(mut))) { \
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); };
#define COND_TIMED_WAIT(cond, mut, microseconds, timeout_result) \
{ \
int r = PyCOND_TIMEDWAIT(&(cond), &(mut), (microseconds)); \
if (r < 0) \
Py_FatalError("PyCOND_WAIT(" #cond ") failed"); \
if (r) /* 1 == timeout, 2 == impl. can't say, so assume timeout */ \
timeout_result = 1; \
else \
timeout_result = 0; \
} \
/* Whether the GIL is already taken (-1 if uninitialized). This is atomic
because it can be read without any lock taken in ceval.c. */
static _Py_atomic_int gil_locked = {-1};
/* Number of GIL switches since the beginning. */
static unsigned long gil_switch_number = 0;
/* Last PyThreadState holding / having held the GIL. This helps us know
whether anyone else was scheduled after we dropped the GIL. */
static _Py_atomic_address gil_last_holder = {0};
/* This condition variable allows one or several threads to wait until
the GIL is released. In addition, the mutex also protects the above
variables. */
static COND_T gil_cond;
static MUTEX_T gil_mutex;
#ifdef FORCE_SWITCHING
/* This condition variable helps the GIL-releasing thread wait for
a GIL-awaiting thread to be scheduled and take the GIL. */
static COND_T switch_cond;
static MUTEX_T switch_mutex;
#endif
static int gil_created(void)
{
return _Py_atomic_load_explicit(&gil_locked, _Py_memory_order_acquire) >= 0;
}
static void create_gil(void)
{
MUTEX_INIT(gil_mutex);
#ifdef FORCE_SWITCHING
MUTEX_INIT(switch_mutex);
#endif
COND_INIT(gil_cond);
#ifdef FORCE_SWITCHING
COND_INIT(switch_cond);
#endif
_Py_atomic_store_relaxed(&gil_last_holder, 0);
_Py_ANNOTATE_RWLOCK_CREATE(&gil_locked);
_Py_atomic_store_explicit(&gil_locked, 0, _Py_memory_order_release);
}
static void destroy_gil(void)
{
/* some pthread-like implementations tie the mutex to the cond
* and must have the cond destroyed first.
*/
COND_FINI(gil_cond);
MUTEX_FINI(gil_mutex);
#ifdef FORCE_SWITCHING
COND_FINI(switch_cond);
MUTEX_FINI(switch_mutex);
#endif
_Py_atomic_store_explicit(&gil_locked, -1, _Py_memory_order_release);
_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
}
static void recreate_gil(void)
{
_Py_ANNOTATE_RWLOCK_DESTROY(&gil_locked);
/* XXX should we destroy the old OS resources here? */
create_gil();
}
static void drop_gil(PyThreadState *tstate)
{
if (!_Py_atomic_load_relaxed(&gil_locked))
Py_FatalError("drop_gil: GIL is not locked");
/* tstate is allowed to be NULL (early interpreter init) */
if (tstate != NULL) {
/* Sub-interpreter support: threads might have been switched
under our feet using PyThreadState_Swap(). Fix the GIL last
holder variable so that our heuristics work. */
_Py_atomic_store_relaxed(&gil_last_holder, (Py_uintptr_t)tstate);
}
MUTEX_LOCK(gil_mutex);
_Py_ANNOTATE_RWLOCK_RELEASED(&gil_locked, /*is_write=*/1);
_Py_atomic_store_relaxed(&gil_locked, 0);
COND_SIGNAL(gil_cond);
MUTEX_UNLOCK(gil_mutex);
#ifdef FORCE_SWITCHING
if (_Py_atomic_load_relaxed(&gil_drop_request) && tstate != NULL) {
MUTEX_LOCK(switch_mutex);
/* Not switched yet => wait */
if ((PyThreadState*)_Py_atomic_load_relaxed(&gil_last_holder) == tstate) {
RESET_GIL_DROP_REQUEST();
/* NOTE: if COND_WAIT does not atomically start waiting when
releasing the mutex, another thread can run through, take
the GIL and drop it again, and reset the condition
before we even had a chance to wait for it. */
COND_WAIT(switch_cond, switch_mutex);
}
MUTEX_UNLOCK(switch_mutex);
}
#endif
}
static void take_gil(PyThreadState *tstate)
{
int err;
if (tstate == NULL)
Py_FatalError("take_gil: NULL tstate");
err = errno;
MUTEX_LOCK(gil_mutex);
if (!_Py_atomic_load_relaxed(&gil_locked))
goto _ready;
while (_Py_atomic_load_relaxed(&gil_locked)) {
int timed_out = 0;
unsigned long saved_switchnum;
saved_switchnum = gil_switch_number;
COND_TIMED_WAIT(gil_cond, gil_mutex, INTERVAL, timed_out);
/* If we timed out and no switch occurred in the meantime, it is time
to ask the GIL-holding thread to drop it. */
if (timed_out &&
_Py_atomic_load_relaxed(&gil_locked) &&
gil_switch_number == saved_switchnum) {
SET_GIL_DROP_REQUEST();
}
}
_ready:
#ifdef FORCE_SWITCHING
/* This mutex must be taken before modifying gil_last_holder (see drop_gil()). */
MUTEX_LOCK(switch_mutex);
#endif
/* We now hold the GIL */
_Py_atomic_store_relaxed(&gil_locked, 1);
_Py_ANNOTATE_RWLOCK_ACQUIRED(&gil_locked, /*is_write=*/1);
if (tstate != (PyThreadState*)_Py_atomic_load_relaxed(&gil_last_holder)) {
_Py_atomic_store_relaxed(&gil_last_holder, (Py_uintptr_t)tstate);
++gil_switch_number;
}
#ifdef FORCE_SWITCHING
COND_SIGNAL(switch_cond);
MUTEX_UNLOCK(switch_mutex);
#endif
if (_Py_atomic_load_relaxed(&gil_drop_request)) {
RESET_GIL_DROP_REQUEST();
}
if (tstate->async_exc != NULL) {
_PyEval_SignalAsyncExc();
}
MUTEX_UNLOCK(gil_mutex);
errno = err;
}
void _PyEval_SetSwitchInterval(unsigned long microseconds)
{
gil_interval = microseconds;
}
unsigned long _PyEval_GetSwitchInterval()
{
return gil_interval;
}